High frequency multiple-channel antenna, particularly for a nuclear magnetic resonance imaging device

Abstract

A multiple-channel antenna includes a first loop resonator including a radiating element forming a loop; a second resonator adjacent to the first loop resonator including a radiating element; wherein the radiating element of the first loop resonator has a main surface defined along a first median plane; a secondary surface defined along a second median plane, the secondary surface delimiting an appendix of the radiating element placed facing the second resonator and having an inclination relative to the median plane of the main surface and wherein the second resonator is a linear resonator with a straight radiating element.

Claims

1. A multiple-channel antenna comprising: a first loop resonator comprising a first radiating element forming a loop; a second resonator adjacent to the first loop resonator comprising a second radiating element; wherein the first radiating element of the first loop resonator includes: a main surface extending in a first median plane; a secondary surface extending in a second median plane, the secondary surface delimiting an appendix of said first radiating element placed facing the second resonator, the secondary surface being oriented with an inclination from the first median plane of the main surface; and wherein said second resonator is a linear resonator with a straight second radiating element.

2. The multiple-channel antenna according to claim 1, wherein the secondary surface delimiting said appendix has smaller dimensions than the dimensions of the main surface.

3. The multiple-channel antenna according to claim 1, wherein a ratio between the secondary surface and the main surface is less than ¼.

4. The multiple-channel antenna according to claim 1, wherein the first radiating element of the first loop resonator has a folding zone delimiting the main surface and the secondary surface.

5. The multiple-channel antenna according to claim 1, wherein the first main surface is a plane or curved surface.

6. The multiple-channel antenna according to claim 1, wherein the secondary surface is a plane or curved surface.

7. The multiple-channel antenna according to claim 1, wherein the angle of inclination α of said appendix relative to the main surface of the first loop resonator is between 45° and 135°.

8. The multiple-channel antenna according to claim 1, wherein said first radiating element of the first loop resonator and said second radiating element of the second resonator are used to transmit a radiofrequency excitation signal and to receive a radiofrequency relaxation signal.

9. The multiple-channel antenna according to claim 1, wherein said first radiating element of the first loop resonator is used to receive a radiofrequency relaxation signal and said second radiating element of the second resonator is used to transmit a radiofrequency excitation signal and to receive a radiofrequency relaxation signal.

10. The multiple-channel antenna according to claim 1, wherein said antenna comprises a third resonator adjacent to the first loop resonator such that the second resonator and the third resonator are placed on each side of said first loop resonator, the first loop resonator having a second appendix placed facing the third resonator and having an inclination relative to the median plane of the main surface.

11. The multiple-channel antenna according to claim 1, wherein the antenna is a high frequency antenna for a nuclear magnetic resonance device.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other characteristics and benefits of the invention will become clearer after reading the following description given for guidance and in no way limitative, with reference to the appended figures among which:

(2) FIG. 1 is a principle diagram showing the coupling phenomenon between a linear resonator and an adjacent loop resonator;

(3) FIG. 2a shows a principle diagram of a first example setup according to an embodiment of the invention for decoupling two adjacent resonators;

(4) FIG. 2b shows a principle diagram of a second example setup according to an embodiment of the invention for decoupling two adjacent resonators;

(5) FIG. 3 shows a simplified example of a first embodiment of a high frequency antenna with multiple channels according to the invention;

(6) FIG. 4 shows a simplified example of a second embodiment of a high frequency antenna with multiple channels according to the invention;

(7) FIGS. 5a and 5b are histograms showing the distribution of reception sensitivities obtained with an antenna according to the state of the art and an antenna according to an embodiment of the invention respectively.

DETAILED DESCRIPTION

(8) FIG. 1 shows a principle diagram showing the coupling phenomenon between a linear resonator 10 and an adjacent loop resonator 20.

(9) Conventionally, the linear resonator 10 is composed of an electrically conducting radiating element 11 called a strip, that is approximately straight and is connected at its centre or at one of the two ends to the external circuit. At high frequency, the strip 11 is usually placed at a given distance (very small compared with the wavelength) from a conducting plane P2 called the ground plane. The strip 11 may be segmented and hold the capacitors in series or it may be connected to the ground plane P2 through capacitors in parallel depending on the length of the strip 11 and the target resonant frequency.

(10) Conventionally, the loop resonator 20 is formed from an electrically conducting radiating element 21, the shape of which matches the contour of a circle or a regular polygon. This radiating element 21, subsequently called a loop, may be broken down into segments connected to each other by capacitors in order to reduce the resonant frequency and make the current distribution more uniform along the loop. The terminals of one of the capacitors act as radiofrequency terminals (RF port) for the connection to the external circuit. At high frequency, the loop 21 is usually placed at a given distance (very small compared with the wavelength and the size of the loop 21) from the ground plane P2. Capacitors may also connect the loop 21 to the ground plane P2 depending on the size of the loop 21 and the required resonant frequency.

(11) When the linear resonator 10 is powered by a generator (not shown), the radiating element 11 called the strip located above the ground plane P2 creates a magnetic field, the field lines of which rotate around the strip in a plane P3 perpendicular to the ground plane P2, the field vector being tangent to these lines.

(12) When the loop resonator 20 is powered by a generator (not shown), the radiating element 21 called the loop creates a magnetic field, of which the field lines 22 are perpendicular to the surface S delimited by the loop 21, according to the principle of the Biot-Savart law.

(13) Placing a loop resonator adjacent to a linear resonator, at about the same elevation from the common ground plane P2, a configuration that occurs for example in array antennas used for medical imaging, field lines 12 generated by the electrically powered strip 11 pass through the surface S of the loop 21 and give rise to a non-zero flux that causes an induced current to appear in the loop 21. This induced current creates a counter-field that tends to oppose the flux variation (Lenz's law). This phenomenon causes transmission of power from the linear resonator 10 to the loop resonator 20. Therefore, there is mutual coupling between the two adjacent resonators.

(14) The antenna disclosed according to an embodiment of the invention has a particular architecture that is capable of cancelling or at least strongly reducing the current induced in the loop 21 to prevent the mutual coupling phenomenon between two resonators without adding an additional component.

(15) FIG. 2a shows a first principle diagram for the antenna installation according to an embodiment of the invention.

(16) The operating principle will be explained based on the simplified principle diagram shown in FIG. 2a that shows a square-shaped loop resonator placed close to a linear resonator.

(17) The linear resonator 110 is a conventional resonator as described above.

(18) The loop resonator 120 is a folded loop with a first plane surface S1 in space called the main surface and a second plane surface S2 called the secondary surface and forming an appendix 122 from the main surface S1. In the simplified example embodiment shown in FIG. 2, the surface S2 forms a right angle from the surface S1. Obviously, the angle of inclination equal to 90° is not limitative and is given only as an example. Similarly, the surfaces S1 and S2 are not limited to plane surfaces.

(19) When the strip 111 is energised, the radiated magnetic field {right arrow over (H)} then passes through the two surfaces S1 and S2 formed by the loop 121, creating a first flux Φ.sub.1 and a second flux Φ.sub.2 defined as follows respectively:
Φ.sub.1=∫∫.sub.S1{right arrow over (H)}.Math.{right arrow over (n.sub.1)}dS,
Φ.sub.2=∫∫.sub.S2{right arrow over (H)}.Math.{right arrow over (n.sub.2)}dS.

(20) The field line that passes through the centre of the surface S1 and the centre of the surface S2 is shown as reference 114, for illustrative purposes.

(21) In general, the resonators operate under harmonic conditions. Therefore, Lenz's law can be used to evaluate the induced currents I.sub.1 and I.sub.2 on the radiating element 121 surrounding the main surface S1 and the secondary surface S2 respectively.

(22) $I_1=-\frac{1}{R}\left(\frac{d{\Phi }_1}{dt}\right)$$I_2=-\frac{1}{R}\left(\frac{d{\Phi }_2}{dt}\right)$

(23) Where:

(24) I is the induced current in amperes;

(25) R is the loop resistance in ohms;

(26) Φ is the magnetic flux in webers.

(27) Assuming that the amplitude of the radiated magnetic field {right arrow over (H)} oriented on reference 114 decreases over a time interval dt, then the special shape of the loop 121 can give a positive induced current I.sub.1 and a negative induced current I.sub.2, considering the conventional orientation of Lenz's law vectors {right arrow over (n.sub.1)} and {right arrow over (n.sub.2)}. The total induced current I in the loop is the sum of currents I.sub.1 and I.sub.2. Furthermore, the amplitude of the magnetic field H decreases as the distance from the strip increases, and is therefore higher on surface S2 than on surface S1.

(28) Consequently, an embodiment of the invention consists of creating a surface S2 distinct from the main surface S1 with smaller dimensions than the dimensions of the main surface S1, with an angle of inclination α from the median plane of the surface S1.

(29) The orientation and position of the surface S2 relative to the surface S1, and the size of the surface S2 relative to the surface S1 are determined such that the amplitudes of the induced currents I.sub.1 and I.sub.2 generated by the presence of the nearby linear resonator 110 are equal, such that the total induced current in the loop resonator 120 cancels out. Thus, the special shape of the loop 121 can decouple two adjacent resonators in a high frequency antenna.

(30) The principle described above remains valid for loops with shapes different from the illustrated square shape.

(31) Therefore, to manufacture an array antenna, a loop resonator 120 should be alternated with a dipole type linear resonator 110, each loop resonator 120 then having two adjacent linear resonators 110. Consequently, the loops 121 must comprises two appendices 122 on each side of the loop 121, the appendices 122 facing the adjacent linear resonators 110 on each side.

(32) For a given strip 111, the appendix 122 furthest from the loop has no significant effect, due to the fast decrease of the field with distance.

(33) Optimum decoupling of an array antenna according to an embodiment of the invention is obtained by an appropriate choice of:

(34) the surface S2 of the appendix that is directly dependent on the height h of the appendix relative to the main surface S1;

(35) the angle α of inclination of the appendix relative to the main surface S1 of the loop.

(36) Beneficially, the ratio of the surface between the secondary surface S2 of the appendix 122 and the main surface S1 of the loop 121 is less than ¼.

(37) For example, FIG. 3 shows an example embodiment of a high frequency antenna with multiple channels 200 according to the invention for examination of a human head in an NMR device. The head is simulated by a spherical test dummy 230 with a radius of 78 mm filled with agar gel, the electrical characteristics of which are similar to the characteristics of organic tissue.

(38) The hybrid antenna 200 alternately comprises a loop resonator 220 with a loop 221 with two appendices 222, and a dipole type linear resonator 210 (only one unit of each resonator being shown in FIG. 3).

(39) The strip 211 of the linear resonator 210 is formed by a 140 mm long, 15 mm wide, and 2 mm thick electrical conductor.

(40) The loop 221 is formed by an electrical conductor forming a total surface S delimited by a 66×66 mm square, the electrical conductor having a 2 mm thick and 4 mm wide rectangular cross-section. For use in a 7-Tesla MRI scanner, the loop resonator 220 comprises four 5.8 picofarad (pF) capacitors that are distributed in series on the loop and are placed at the middle of each of the sides of the loop, so as to adjust the resonant frequency to 298 MHz. For this configuration, the height of the appendix 222 that cancels out the total current induced in the loop 221 with dimensions 66×66 mm is between 12 and 13 mm.

(41) By using such a configuration, the transmission coefficients between the loop 221 and the strip 211 that characterise mutual coupling are between −30 dB and −27 dB. For an appendix height of 12 mm; the average field B.sub.1.sup.+ in the test dummy 230 that indicates the efficiency of a resonator in transmission is about 0.25 microTesla (μT) for an incident power of 1 W, both for the loop 221 and for the strip 211.

(42) For comparison, if a linear resonator was used instead of the loop resonator described above, or if a loop resonator according to the state of the art was used without the appendices, mutual coupling between the two resonators would be significantly more than −6 dB, −6 dB already being a very poor value in terms of adjustment and performance of the antenna.

(43) By targeting mutual coupling with a value of less than −20 dB, the resolution with which the parameter h (height of the appendix) must be defined is of the order of one millimeter (mm). In practice, it will be impossible to physically modify the appendix to adjust the height h once the appendix has been formed. The modification of the angle of inclination α of the appendix with the main surface S1 of the loop can advantageously be used to modify and adjust mutual coupling so as to minimise it. This operation may be done in various ways; for example by the choice of a conductor cross-section compatible with bending; by placement of hinges with locking elements between the appendices and the main surface S1.

(44) According to one variant embodiment, conductors forming radiating elements of loop resonators may be supported and stiffened by stiffening parts made of low-loss dielectric materials.

(45) The angle of inclination α of the appendices relative to the main surface S1 is advantageously chosen to be between 45° and 135°. Such an inclination interval has a relative variation of the height h equal to ±30%. Thus, the range of variation of the inclination of the appendices is easily sufficient to minimise mutual coupling between two resonators. However, this analysis assumes that the magnetic field does not vary much in the vicinity of the appendix. This condition is satisfied when the height h is of the order of 1% of the wavelength. If the loop resonator has two appendices, these appendices may have different inclinations, particularly as a result of final adjustments necessary to reach minimum mutual coupling.

(46) Beneficially, the linear resonators and loop resonators described in this description are used both in transmission and in reception so as to optimise operation of the antenna.

(47) However, in some applications, it may be sufficient to use only linear resonators in transmission and all resonators (linear and loop) in reception.

(48) In the example embodiment of the array antenna according to the invention, linear resonators may be placed setback from the loop resonators. For example, the strip is beneficially placed at mid-height of the appendix. An offset of the position between the linear resonators and loop resonators makes it possible for example to respect specific absorption rate (SAR) requirements when only linear resonators are used in transmission.

(49) The second example embodiment shown in FIG. 4 is an array antenna 300 with twelve linear resonators 310 distributed in two groups of six resonators 310 placed offset along the longitudinal direction of the antenna 300. With a technology according to the state of the art, the increase in the number of resonators would increase the mutual coupling unacceptably. With an embodiment of the invention, the number of resonators can be increased by adding ten loop resonators 320 according to an embodiment of the invention, and potentially twelve if there are no windows 312 dedicated to visual stimulation used in functional MRI.

(50) It is convenient to use the reciprocity principle to evaluate the reception sensitivity of an array antenna in MRI using digital simulation, and to consider the distribution of B.sub.1.sup.− when a resonator is powered by an incident power of 1 W. The sensitivity associated with the reconstruction of images using the “sum of squares” method for an array antenna with N resonators, is defined by the following relation:

(51) $S=\underset{i}{\overset{N}{.Math.}}{.Math.B_{1i}^{-}.Math.}^2$

(52) Therefore, it is easy to quantify the gain in sensitivity in reception obtained by the addition of 10 decoupled loops according to the invention, by evaluating S at each voxel of the sample using maps of field B.sub.1i.sup.− of strips alone or the combination of strips and loops.

(53) The results presented below correspond to maps obtained by digital simulation using the ANSYS HFSS program and an anatomic dummy with two layers: a muscular envelope 313 and an ellipsoid-shaped brain 314. The electrical relative permittivity and the electrical conductivity are 59.5 and 0.78 S/m respectively for the first layer, and 45.3 and 0.87 S/m respectively for the second layer.

(54) The simulations made show an average gain in the ellipsoid of 49%, between the sensitivity in reception of strips alone (state of the art) and the sensitivity of strips combined with loops according to the invention. This gain is not uniform on the three characteristic planes in the ellipsoid-shaped brain. The highest gain is obtained in zones in which the sensitivity in reception of strips alone was lowest, which is in the upper part of the skull. The gain locally reaches 97% in this zone.

(55) The distribution of reception sensitivities in the voxels is shown by the histograms in FIGS. 5a and 5b. The histogram in FIG. 5a shows the distribution of reception sensitivities in voxels for an array antenna according to the state of the art using linear resonators only. The histogram in FIG. 5b shows the distribution of reception sensitivities in voxels for a hybrid array antenna according to an embodiment of the invention shown in FIG. 4. Thus, the histograms show that the average sensitivity is 0.99 μT for the hybrid array (i.e. strip and loop) according to an embodiment of the invention compared with 0.67 μT for a strip array alone. Furthermore, the zones in which the sensitivity in reception is less than 0.5 only contain 2% of the voxels for the hybrid array while they contain 33% for the array with strips only. The invention has been described particularly for use in nuclear magnetic resonance imaging; however the invention is equally applicable to other application fields using a high frequency multiple-channel antenna operating in near field.

(56) The invention has been described particularly for a loop resonator with a radiating element with a first plane main surface and two plane appendices on each side of the main surface. However, the invention is equally applicable with a loop forming a curved first main surface (i.e. with at least one radius of curvature) defined by a median plane and/or two curved appendices (i.e. with at least one radius of curvature) also defined by a median plane.

(57) FIG. 2b shows a second principle diagram for assembly of the antenna according to an embodiment of the invention which is a variant of the first principle shown in FIG. 2a.

(58) In this variant embodiment, the linear resonator 110 is replaced by a loop resonator 410 formed from an electrically conducting radiating element 411 with a shape that matches the contour of a circle or a regular polygon. The loop resonator 410 is a conventional resonator known to those skilled in the art, as described above in FIG. 1 with reference 20.

(59) In this variant embodiment, the dimensions of the loop resonator 410 are determined such that the magnetic field created by the edge B, in other words the edge furthest from the folded loop resonator 120 and experienced by this folded loop resonator 120, is negligible compared with the magnetic field created by the edge A, in other words the edge closest to the folded loop resonator 120 and experienced by the folded loop resonator 120. For example, it can be assumed that the magnetic field created by the edge B and experienced by the folded loop resonator 120 is negligible when the distance between the two edges A and B is equal to at least five times the distance between the two edges of the folded loop resonator 120.

(60) The invention has been described particularly with reference to an array antenna comprising a loop resonator with two appendices alternating with a dipole type linear resonator. However, in some applications, it is also envisaged that an array antenna could be made comprising a loop resonator with two appendices as described in this application alternating with a loop resonator according to the state of the art (i.e. without appendix), with a width equal to at least five times the width of the folded loop resonator.